Electroacoustic transducer for actuating loud speakers



Jan. 16, 1951 M. MORSE 2,538,026

ELECTROACOUSTIC TRANSDUCER FOR ACTUATING LOUD-SPEAKERS Filed May 7, 19462 Sheets-Sheet 1 I 5 18 jg v Q6 29 i :2 J6 19 1 4L I/ "3 J5 I I; fififfiof Ni INVENTOR M11507; M01198 EY I v I I I ATTORNE Jan. 16, 1951M. MORSE 2,533,025

ELECTROACOUSTIC TRANSDUCER FOR ACTUATING mmxsrmxrzas Filed May '7, 19462 Sheets-Sheet 2 TIQE.

CONVERSION ABILITY 2 25 s 4 5 INVENTOR SPACING'FN TERMS OF AVERAGE Mgzggjlfons'e GRAIN DSAMETER BY fromxg Patented Jan. 16, 1951 ELECTROACOUSTICTRANSDUCER FOR .ACTUATING LOUD SPEAKERS Milton Morse, New York, N. Y.,assignor to University Loudspeakers Inc., New York, N. Y., a corporationor" New York Application May '7, 1946, Serial No. 667,964

3 Claims. 1

My present invention relates generally to the transmission of sound, andhas particular reference to electro-mechanical transducers.

It is a general object of the invention to provide a carbon-graintransducer of improved design and enhanced conversion ability, i. e.,ability to transform mechanically applied energy into faithfullyresponsive electrical energy.

The broader phases of my invention are applicable to various purposes,depending upon the source and nature of the applied energy. For example,the energy applied may be derived from a grooved record, as in the caseof a phonograph pick-up, or it may consist of acoustic energy in theform of sound waves transmitted through the air, as in the case of amicrophone. Because of its particular value and importance in the fieldof microphones, I have chosen herein to describe and illustrate thenature and advantages of the invention in the embodiment of a microphoneand a microphone-loud speaker circuit.

As is well known, a carbon-grain micro-phone consists essentially of apair of electrodes and a mass of carbon grains (or equivalent material)interposed between them, one of the electrodes (commonly referred to asa diaphragm) being mounted for vibration in response to sound-waveimpulses impressed thereon. When inserted in series in a direct-currentcircuit, the vibrations of the diaphragm efiect corresponding variationsin the carbonaceous resistance path between the electrodes, and thecurrent flowing in the circuit is thus caused to fluctuate in responseto the sound-wave impulses.

In conventional micro-phones, the design is of such character that theaverage resistance is of relatively high order, usually eighty ohms ormore, and the current transmitted is correspondingly minute. Such acurrent is adequate for the purpose of actuating an earphone, such as atelephone receiver, but requires considerable amplification in order tooperate a conventional loud speaker. Accordingly, in public-addresssystems and thelike, the necessary equipment has always heretoforeincluded not only the microphone and the loud speaker, but also arelatively expensive and cumbersome amplifying apparatus as well.

In accordance with my invention, I am enabled to provide a microphonehaving an average resistance of such low magnitude that it is capable oftransmitting a low-voltage current which is adequate, withoutamplification, to actuate a loud speaker of comparably low resistance.By

low voltage I intend to signify a voltage of the order or six volts,such as might be conveniently furnished by one or a few small dry-cellbatteries or by an automobile storage battery, or the like. By lowresistance I mean a resistance within the range of one to fifteen ohms,such as is manifested by the conventional loud speaker of a radioreceiving set or a so-called publicaddress system.

So far as I am aware, no microphone of conventional or practical size orcharacter has ever been made in which the average resistance is lowerthan about eighty ohms, and which has sufficient electro-acousticconversion ability to adequately operate a loud speaker for publicaddress purposes without amplification. This is due in part to the factthat a lower resistance would be unnecessary and undesirable intelephone circuits or the like, where a correspondingly high lineresistance exists, and also to the electrical and mechanicaldifficulties which are encountered and which have not heretofore beensuccessfully overcome.

The achievement of the present improved resuits is predicated upon astudy of the factors influencing microphone resistance, currenttransmitting capability, and conversion ability, and upon the discoverythat certain hitherto unknown or unrecognized critical relationshipsexist where relatively low average resistances are involved.

It is known that the impedance of a carbongrain microphone is a functionof the average grain size, of the operative electrode area, and of thespacing between electrodes. Thus, generally speaking, an increase ingrain size will lower the resistance, whereas a decrease in grain sizewill raise it. In the conventional microphone the grains employed areusually of a size considerably smaller than 50-mesh (one fiftieth of aninch) and are most commonly of the order of IOU-mesh to ZOO-mesh.Similarly, it may be stated as a general proposition that an increase inthe spacing between the electrodes will increase the resistance, whereasa reduction in the spacing will decrease it; and in a smaller measure,an increase in operative electrode area (exposing the grains to a largercontact area) will decrease the resistance, while a reduction of sucharea will increase it.

With the object in view of having the microphone transmit, at lowvoltage, a current of suflicient magnitude to actuate a loud speakerwithout any preliminary amplification, it becomes important to establisha matched impedance between the microphone and the loud speaker, and theproblem resolves itself to the design of a microphone having aresistance of less than fifteen ohms, which is the resistance mostcommonly manifested by the average loud speaker.

In achieving this object, reliance cannot be placed solely on anenlargement of the operative electrode surface without entailingmechanical and other dimculties incident to impractically largedimensions. Recourse is therefore preferably had to an enlargement ofthe ratio between the grain size and the spacing between electrode Toavoid the practical difficulties involved in providing for andmaintaining too minute dimensional relationships between the electrodes,this involves an enlargement of the grain size.

My experiments have shown that as the ratio between grain size andelectrode spacing increases toward unity, there is a criticalrelationship between grain size and electrode spacing which must bemaintained in order to avoid such impairment of conversion ability as todefeat the desire'd obj ective. Moreover, at the critical ratio,theconversion ability has been found to be of surprisingly goodenlciency, considerably greater than any conversion ability heretoforepresent in conventional microphones.

It is therefore a feature of my invention to provide a carbon-grainmicrophone in which the operative electrode area, the average graindiameter, and the spacing between electrodes, are so chosen that theremay be achieved not only the desired low average resistance but also aconversion ability of high order. This result is capable ofaccomplishment, in accordance with my invention, by a special design inwhich the spacing between the electrodes is caused to be no less than2.0 and no more than 2.6 times the average grain diameter. Thus, wherethe grain size is in the preferred range of 50-mesh to lG-mesh (onetenth of an inch), the spacing between the electrodes is caused to liebetween the dimensional values of 0.04: and 0.26 inch. A thoroughlypracticalconstruction lying within this range, and involving an over-allmicrophone size or" conventional character, employs grains ofsubstantially uniform size whose average diameter is 6.95 inchQZO-mesh), and has the electrodes mounted between 0.10 and 0.13 inchapart.

It is also important that the vibrations of the diaphragm be unimpededby grain-retainin washers or similar structure, and, to th contrary,that the vibratable electrode be as free as possibl to move in responseto the sound-wave impulses impressed upon it. This I achieve by aspecial construction and assembly of parts, involving the employment ofcomplementary electrodes which are of generally concave convexconfiguration, the convex electrode having a base which defines acylindrical contour. By encircling this base with a collar which isseparate'd from the base by a clearance gap, the convex electrode isleft entirely free to vibrate in the direction of the cylinder axis, andat the same time the clearance gap may be made small enough, relative tothe grain size, to prevent escape of the grains through it.

A further feature of the invention resides in the provision of a novelunitary structure which not only supports the microphone in anefiicient, preferably pivotable, relation to a handle, but serves alsoas a convenient means for accommo-- dating one or more low-voltagedry-cell batteries with which the microphone is to be connected inseries.

The preferred way of achieving these general objects and advantages, andsuch other objects and advantages as may hereinafter appear or bepointed out, is illustratively exemplified in the accompanying drawings,in which:

Figure l is a diagrammatic representation of an illustrativesound-transmitting circuit to which the invention may be applied;

Figure 2 is a longitudinal cross-sectional view through a microphone ofthe present improved design, shown as part of a unit including abattery-accommodating handle;

Figure 3 is a cross-sectional view taken substantially along the line3-3 of Figure 2, including the rear or free end of the handle;

Figure 4 is an enlarged fragmentary cross-section taken substantiallyalong the line 34 of Figure 2;

Figure 5 is a perspective view of the entire microphone-handle assembly,showing its relation to a connecting cord of conventional type; and

Figure 6- is a graph depicting certain relationships observed byexperimentation.

One of the desirable objectives achieved by the present invention isdepicted in Figure 1., in which I have diagrammatically represented amicrophone it] connected in series with a conventional loud speaker II,and with a source of low voltage direct current l2. It will be observedthat this circuit is devoid of the conventional amplifier which has beenregularly employed as a n cessary accessory in the circuit between amicrophone and a loud speaker.

The microphone Ill and the battery [2 may be conveniently assembled inthe form of a unitary structure, as shown in Figure 5. In a mannerpresently to be described in greater detail, the microphone it issecured to one end of a cylindrical handle i3 adapted to accommodate oneor more simple dry cell batteries. At the rear or free end of the handlel3, recesses are provided for the purpose of removably receiving aconventional two-pronged connector l4 electrically secured to the end ofa flexible cord [5, By connecting one end of the cord IE to anyconven-'v tional loud speaker, and the other end tothe handle it, a,sound-transmitting equipment is available, consisting in its entirety ofthe elements mentioned, and requiring no cumbersome or expensiveamplifying apparatus. The advantages of this simplified arrangement willbe apparent to those conversant with the art of ordinary so-calledpublic-address system.

The microphone of the present invention is shown most clearly in Figure2. A metal housing is provided, preferably of substantially cylindricalshape, and consisting (in the illustrated embodiment) of a forwardelement ie'and a rearelement ii, the latter being fitted snugly over areduced rearwardly-directed skirt on the element [6. The elements [6 andll may be composed, for example, of aluminum or the like, and may bedie-cast, stamped and drawn, or formedin any other convenient manner.

'Snugly fitted within the portion '6' is a .sup.

23 is clamped the marginal portioii'of aninner front wall 24. This wall,and the outer front wall 25 (formed, preferably, as an integral part ofthe housing element it), constitute the front of Preferably, each ofthese walls is provided with a series of openings 2% to permit thepassage of the sound waves. The walls and 25 are commonly referred to asacoustic filters. Both walls are preferably composed of aluminum or thelike, and the spacers 2i and 2'5 may be composed of similar material.

Clamped in position behind the inner acoustic filter 24, I provide aflex ble moisture-proof barrier 2?, which may be composed, for example,of rubber, rubberized silk, Vinylite film, or the like, approximatelytwo-thousandths of an inch thick. As will be well-known to those skilledin the art, this wall is of limp and unstretched character and servesmerely to prevent the passage of undesirable moisture to and through thediaphragm 22.

The diaphragm 2?; is of more or less conventional character, beingcomposed of aluminum, aluminum alloy, or the like, and beingapproximately two-thousandths of an inch thick. It has a slightly bowedshape, as shown, except that its marginal portion lies flat in a singleplane in clamped engagement between the parts 59 and 2|. Adjacent tothis margin, the diaphragm 22 may be provided with the well-knowndiagonal fiutings 28 (not shown in detail) which enhance the freedom ofvibration of the central bowed portion under the impact of the soundwaves which pass through the filters 25 and 2t, and through themoisture-proof diaphragm 2?. The diaphragm 22 may be provided, ifdesired, with one or more minute holes 221 to prevent any entrapped airto dampen or impede the desired free vibration of the diaphragm.

As is well known, it is the central portion of the diaphragm whichserves as the operative electrode. I prefer to employ a convex electrode36 of substantially hemispherical shape, this element being composed ofbrass or copper, plated with gold or platinum. This element isapproximately two-thousandths of an inch thick and is mounted inassociation with the diaphragm by initially providing the latter with acentral opening and by forming flanged fingers on the rear margin of theelement 39, for clamping engagement with the margins of the hole in thediaphragm; These fingers have not been shown in the present drawings, inwhich, for the sake of convenience and simplicity, the convex electrode33 has been shown as an integral part of the diaphragm 22 as a whole.

For a purpose presently to be described, the base of the electrode i.e., the portion adjacent to its point of connection with the diaphragm22, is purposely caused to have a cylindrical contour. This cylindricalshape is best observed in Figure 4.

The vibratable electrode 3t cooperates with a complementary concaveelectrode which I have shown in the form of a cup'shaped plug 3! mountedwithin the bore 28 of the part l8, and insulated from the latter bymeans of a cylindrical sleeve or bushing 32 of insulating material, suchas Bakelite or the like. The part 3| is composed of copper or brass, andits concave face is plated with gold or platinum.

In the space between the concave face of the electrode 3| and the convexouter face of the electrode 3s, a mass of carbon grains 33 are arranged.They may be inserted and withdrawn from this space through a centralopening in the element 3|, which is plugged by a threaded element 34.The latter is accurately constructed and machined so that when it is inposition, as shown, its inner face' is concave and constitutes acontinuing portion of the concave operative face of the electrode 3|.With this objective in view, the plug 34 is preferably composed ofcopper or brass, and its innerconcave end is plated with gold orplatinum.

The carbon grains 3t are kept within the space between the electrodes bya retainer lip or 001-,

lar 36 which encircles the cylindrical base of the electrode as andprojects part-way across the annular inner end of the space within whichthe carbon grains are accommodated. This relationship of parts is shownmost clearly in Fig-i ure 4, and it will be observed that there is aclearance gap 31 between the collar 36 and the cylindrical base of theelectrode 3! permits free vibration of the electrode S ll in thedirection of the cylinder axis, yet it may be made small enough,relative to the average size of the carbon grains employed, to preventescape of the latter through it. For example, where the carbon grainshave an average diameter of 0.05 inch, the clearance gap iii may beapproximately 0.01 inch, even slightly larger than this.

The unique capabilities of the present micro-- phone and microphonecircuit are predicated upon the relationships which experimentation hasshown to exist and which are depicted in the graph of Figure 6.

If the vertical axis of this graph is caused to represent the conversionability of the microphone, while the horizontal axis represents thespacing between the electrodes 35] and 3|, in terms of the average graindiameter, it will be understood, at the outset, that the ratio betweengrain diameter and spacing cannot be less than unity. Under thesecircumstances, there would be a single layer of carbon grains betweenthe two electrodes, and vibration of the electrode 311 would be entirelyprecluded, except such minute movement as may be permitted by thecompressibility of the carbon grains. these circumstances is practicallynil, as indicated by the portion 33 of the graph. As the spacing betweenthe electrodes is increased, the conversion ability is found to risesuddenly and markedly as the spacing reaches a magnitude twice as greatas the average grain diameter. Surprisingly enough, however, a slightincrease of the spacing beyond a value approximately 2.3 times theaverage grain diameter causes an almost equally sudden falling off ofconversion ability, as represented by the portion 39 of the graph. At aspacing almost equal to three times the average grain diameter, theconversion ability rises slightly, as indicated at it, and as thespacing is gradually increased beyond this amount,

the conversion ability diminishes rather grad ually, as indicated by theportion ii of the graph.

It is my theory that the behavior of the microphone so far aselectro-mechanical conversion ability is concerned, is explained asfollows: When the space between the electrodes is less than two timesthe average grain diameter, the carbon grains tend to bind or freeze,thereby seriously impeding, if not entirely preventing, free vibrationof the vibratable electrode or diaphragm. At or slightly beyond anelectrode spacing which is twice the average grain diameter, an optimumrelationship of parts manifests itself, wherein an effective currentpath exists having a resistance of minimum magnitude but of maximumvariability. This path extends through a maximum of two grains, one ofwhich establishes a low-resistance contact with one of the gold- Thisgap- Conversion ability under assaoze plated'or platinum-platedelectrodes, the other of which establishes a similar low-resistancecontact with the other gold-plated or platinumplated-electrode. Thepoints of contact resistancere thus only three in number, two at theelectrodes, the third between the two carbon grains. In the range ofspacing depicted by the portion 39 of the graph, however, the ratiobetween electrode spacing and grain size becomes sufiiciently large forgrain slippage and added inter-grain contacts to cause a loss inelectro-mechanical conversion ability. Thereafter, as the spacingincreases, the physical weightof the mass of carbon grains begins tomanifest itself by a loading of the vibratable electrode, and thesensitivity of the microphone is thereby impaired. Moreover, as thespace between the electrodes increases, the resistance of thecarbonaceous current path increases, and this also lowers enicienoy andimpairs conversion abality.

Another reason for the extremely high conversion ability in the peakedportion of the graph of Figure 6 lies in the arrangement of the carbongrains. In the range in which the electrode spacing is between 2.0 and 2.6 times the average grain diameter, and especially in the range between2.1 and 2.3, the grains group themselves in such a way as to form anassembly which is stable (not readily rearranged) and which neverthelessdoes not pack or freeze. This type of grouping results in maximumtranslation of the energy imparted to the moving electrode intoeffective variations in pressure between carbon grains, these pressurevariations being (as is well known) the basic cause of conversionability.

By way of comparison, when the movable electrode of an ordinarymicrophone receives an impulse of movement, it causes a new arrangementof carbon grains by the sliding of one grain over another, and it alsocauses additional compression between the grains. It is only thecompression which results in efiective conversion ability; the slidingof the grains over one another causes only undesired noise and 'noelectro-acoustic conversion. In the conventional type of carbonmicrophone, the greater portion of the energy applied to the'movingelectrode is wasted in this fashion. In the present microphone, on theother hand, this waste of energy, due to sliding of the carbon grains,is minimized if not entirely eliminated. In the peaked portion of thegraph of Figure 6, especially at its upper end, the carbon grainsarrange themselves in such a manner that any pressure on the movingelectrode serves mainly to cause effective variation in compressionbetween the grains. that the size relationship makes it impossible forany substantial amount of inter-grain slippage to occur.

It is also a feature of my invention that in the range represented bythe peaked portion of the graph, there is no jamming or freezing of thecarbon grains into a rigid mass. essary to employ any special springs orcompressible electrodes to avoid such a result.

In conventional microphones, the conversion ability lies along theportion 4%! of the curve depicted in Figure 6, since the ratio of grainsize to electrode spacing is well below one to three, i. e., the spacingin terms ofaverage grain diameter is well over three.

The behavior of the carbon grains, and of the resultant conversionability of the microphone, in the range where the ratio between graindiameter-:and'electrode spacing increases toward unity This is due tothe fact It is thus unnec' All 8 l has led me to the inevitableconclusion, corroborated by experiment, that a reliable andstablemicrophone of low impedance can be constructed only if the electrodespacing is such, relative to the average grain size, that the effectivecurrent paths extend through only two grains, i. e., each current pathof minimum resistance involves no more than one carbon to-carboncontact. This, I have found, can best be achieved only if the electrodespacing lies within the critical range between approximately 2.0 and 2.6times the average grain diameter. Within this critical range, theconversion ability of the microphone is unusually good. The preferredspacing between the electrodes is apparent from an inspection of Figure6 and lies within the dimensional range between 2.1 and 2.3 times theaverage grain diameter.

In this connection, it should be observed'that the provision of metallicelectrode surfaces having good conductive properties, as by thegold-plating or platinum-plating hereinbefore mentioned, is ofoutstanding importance in a construction of the present character; sincethe contacts between the electrodes and the carbon constitute two-thirdsof the total number of points of contact resistance, whereas in theconventional microphone having a much smaller ratio between grain sizeand electrode spacing, these points of contact play only a minor role inthe total resistance of each effective current path.

It shoul' also be noted that the term grains as used herein is intendedto refer generally to cornminuted material obtained by crushing orgrinding a conductive material such as carbon, whether or not thematerial ground or crushed is originally of irregular configuration, insheet form, or in the form of rods or other shapes, and whether or notthe resultant grain is truly or approximately spherical, or iscube-shaped, of elongated rice-like contour, or otherwise contoured orproportioned. The term grain diameter is intended in each case to referto'the dimension as limited by the mesh through which the particlespass; i. e., a grain of 20-mesh diameter (regardless of its shape) isone which will have been held back by a smaller screen but which passesthrough a screen having square openings arranged twenty to the inch.

In the light of this observation, it will be understood that thespherical shape shown at 33 in the present drawings are purelyillustrative and are not to be construed as restricting the constructionor the basic ideas involved to the use of carbon grains having thisparticular configuration. Moreover, while I have, for the sakeofbrevity, designated the grains as being carbon grains, this term isintended to include within its significance all other materials ofequivalent properties suitable to serve the same purpose. Such materialsinclude, for example, pure elements such as silicon and selenium, aswell as combinations of metallic and non-metallic substances which aresimilarly conductive to electric current and have the property ofmanifesting s stantial variations in resistance when .the pressure ofcontact between grains is caused to vary.

In the illustrative construction depicted in Figure the parts aredeliberately designed in such a way that the aforementioned criticalrelationship is maintained. In the first place, I employ grains ofsubstantially uniform size, whose average diameter is between 50-meshand 10- mesh, the preferred size being ZO-mesh. In the second place, theparts are so designed that the spacing between the electrodes 30 and 3|will be between 2.1 and 2.3 times the average grain diamto theelectrodes are not intended to restrict the construction to one in whichthe electrode surfaces are truly spherical in contour, or curved at all.It is within the purview of the invention, for example, that theelectrodes might have opposed operative surfaces which are conical orpyramidal in contour, or composed of plane facets arranged at angles toone another. In any case, the one which protrudes may be said to beconvex, in a broad sense, relative to the complementary electrode whichis correspondingly concave.

Where the electrodes are hemispherical, as shown in the present drawingsfor illustrative purposes, the preferred size of the operative electrodearea is achieved when the convex electrode 30 has an outside radius ofabout 0.1875 inch. Under these circumstances, using a ZO-mesh grain, theconcave face of the electrode 35 has a radius of approximately 0.2925 to0.3025 inch. These radii, i. e., of the convex and concave electrodeareas, may be increased or decreased, depending upon the use to whichthe microphone is to be put. On the increasing side, however, a limit isreached when the inertia of the vibratable electrode becomes too greatto respond actively to the ordinary human voice. Generally speaking, ifthe microphone is to be used for a public-address system, which callsfor relatively large wattage, the radii may be of the illustrativedimensions given; and where the microphone is to be used for a purposewhere a smaller wattage is needed, as in an inter-omce communicatingsystem or the like, the radii of the convex and concave electrode areasmay be correspondingly reduced. An aid to power consideration issometimes achieved by using a concentrator or mouth piece on themicrophone.

It is contemplated that the microphone will be inserted in a circuit inwhich there is a 1owvoltage source of direct current, such as six voltsor so. While the source of direct current may be of any suitablecharacter, such as the storage battery of an automobile, it frequentlybecomes desirable to utilize the present microphone in a circuit inwhich the source of power is a simple dry cell battery. In such a case,the unitary assembly shown in Figure 5 is of unique advantage. Apreferred relationship of the parts is shown in detail in Figures 2 and3.

Referring to these figures, it will be observed that I have provided thehandle IS in the form of an elongated cylinder, preferably of metal suchas aluminum or the like. In its top wall .2 I provide a central aperturein which an insulating bushing 3 is mounted. Projecting through thisbushing is a contact element 34 whose inner end is flattened as at 45,whereby it is adapted to establish electric contact with the center pole=15 of a conventional dry cell battery 4'! which fits snugly butslidably within the handle i3.

On opposite sides of the contact element M I provide two spacedupstanding ears 48 on the end wall 42, through which a pivot pin 49extends. On the housing of the microphone I provide a similar pair ofears 50 which also fit over the pivot pin 49. This establishes a pivotalconnection between the microphone housing and the handle, and byarranging the parts in suitable frictional relat onsh p the mic phonemay be readily adjusted into varying angularities with respect to thelongitudinal axis of the handle l3.

Projecting from the microphone is a contact element 5! similar to theelement M. It extends into a bore having a lining 52 of insulatingmaterial, this bore extending all the way to the electrode 3|. A coilspring 53 is mounted in the inner end of this bore, and serves toestablish an electric connection between the electrode 3! and thecontact element 5|, at the same time serving yieldably to urge theelement 5|. outwardly.

Mounted on the pivot pin 48, in insulated relation to it, by means of aninsulating sleeve or bushing 54, is a bridging contact element 55 havinga peripheral contact surface which is concentric with the pivot axis andwhich establishes contact with both the elements 5| and 14, regardlessof the angular position into which the microphone is set. I

At the rear end of the housing [3, an end plate 56 of insulatingmaterial is fitted, preferably being held in place by friction. It isprovided with two spaced apertures 51 into which the usual two prongs 58of the connector l4 (Figure 5) may be inserted. Within the handle l3,and aligned with the apertures 51, are the connection terminals 59 and60 adapted to engage with and establish electrical contact with theprongs 58, respectively, when the latter are inserted. The terminal 59is formed-on a ring-shaped disk 6| which is clamped inposition by theplate 56, and establishes a permanent electrical connection with thebody of the handle 13, hence with the ears 48, 50, the microphonehousing, and the vibratable electrode 30. The other connection terminal50 is mounted on a centrally positioned stem 62 on which a conductivesleeve 63 is mounted, this sleeve having a flattened end 64 pressed by acoil spring 65 into constant electric contact with the rear pole 66 ofthe battery 61. The stem 62 is preferably threaded on its interior, sothat it may be secured to the plate 55 by the screw 68.

The handle [3 may be of any desired length, so that one, two or more drycell batteries, such as those indicated at 4! and 61, may beaccommodated in tandem relationship. Where four batteries of 1 voltseach are used, the total voltage available in the circuit is obviouslythe 6 volts for which the microphone is primarily intended.

From the description given, it will be obvious the the electrode 30 isin electric contact with the microphone housing, and that there is aninsulated electric contact established, by the contact elements 44 and51, between the insulated electrode 3| and the center pole of thebattery. Similarly, it will be observed that one of the connectionterminals, 1. e., the terminal 60, is in electric contact with the rearpole of the battery, and the other terminal 59 is in electric contactwith the microphone housing, whereby the insertion of the plug I4 intothe handle [3 completes the electric circuit diagrammatically shown inFigure 1.

In general, it will be understood that many of the details hereindescribed and illustrated may readily be modified by those skilled inthe art without necessarily departing from the spirit and scope of theinvention as expressed in the appended claims. Except as otherwisespecified in the claims, therefore, it is intended that these details beinterpreted as illustrative, and not in a limiting sense.

Having thus described my invention and illusbration of said first-namedelectrode in the di- 1 rection of said cylinder axis and which is smallenough, relative to the average grain size, to prevent escape of saidgrains through it.

2. In an electro-mechanical transducer as defined in claim 1, and one ofsaid electrodes being vibratory and of dome shape, the other of saidelectrodes being fixed and having a concave recess for receiving saidvibratory dome-shaped electrode and uniformly spaced therefrom toreceive the carbon grains, said fixed electrode hav- 4 ing a plug whichis removable to permit access to the space between the electrode for thepurpose of filling the same with the carbon particles, the end of saidplug being concaved to conform to the surface face of the concaved fixedelectrode whereby to maintain the uniformity of the spacing of theelectrodes.

3. A carbon-grain microphone comprising a pair of spaced electrodeshaving carbon grains of predetermined size between them, one of saidelectrodes being vibratory and having a dome facing the other electrode,said other electrode being fixed and being concaved to receive saidvibratory electrode, means for mounting said vibratory electrode, saidmounting means enabling said electrode to vibrate in response tosoundwave impulses impressed thereon, the operative electrode areaadjoining the grains, the grain size, and spacing means between saidelectrodes being sufiicient to produce an average resistance paththrough the grains from one electrode to the other being of relativelylow order of less than fifteen ohms, whereby the microphone can transmitat low voltage a fluctuating current which is responsive to saidimpulses and adequate in magnitude, without amplification, to actuate aloud speaker of comparably low resistance.

MILTON MORSE.

REFERENCES CITED The following references are ofrecord in the file ofthis patent:

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